1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, more particularly, to a lithium-ion secondary cell having a total size of less than about 5 cm3 and which is capable of being pulse discharged.
2. Prior Art
Secondary cells, which shuttle lithium between the anode and cathode, are well know in the art. These cells, typically referred to as lithium-ion cells, have a negative electrode of a carbonaceous anode material and a positive electrode of a porous cathode active material composed of a lithiated metal oxide. Cells of this type are widely available commercially and are typically rated up to a 2C constant current discharge. The typical coating thickness for the anode and cathode active materials of the respective negative and positive electrodes for these prior art cells is on the order of 0.08 mm per side of current collector. This means the electrodes are usually greater than about 0.15 mm thick. Such coating thicknesses result in secondary cells having less coated current collector surface area, higher internal resistance, less pulse power and longer total cell volume than that of the present invention secondary cell. In part, lithium-ion cells having electrodes of a thickness greater than about 0.15 mm are not able to provide enough surface area to be housed in a casing having a total external volume less than about 5.0 cm3 and to be pulse discharged.
For example, U.S. Pat. No. 5,411,537 to Munshi et al. describes a xe2x80x9cAAxe2x80x9d size rechargeable cell for powering a bioimplantable device. The inventors of the present invention secondary cell simulated the electrode design and discharge performance of various lithium rechargeable chemistries based on the Munshi et al. patent. In order to satisfy the power requirements of an implantable medical device, it was determined that secondary cells according to Munshi et al. require a casing volume which is much larger than the 5 cm3 total size of the present secondary cell. In fact, the Munshi et al. cell is described as weighing 18 grams and having a total volume of 8 cm3.
Accordingly, the present invention provides a rechargeable lithium-ion cell capable of being discharged to deliver high power pulses sufficient for implantable defibrillator applications. Another objective is to provide a secondary cell which is less than about 5 cm3 in total volume. Still another objective is to provide a secondary cell with minimal self-discharge, as required for implantable medical device applications.
The rechargeable cell of the present invention comprises a negative electrode of a carbonaceous material or a material capable of forming an alloy with lithium. The cathode active material of the negative electrode is preferably a metal oxide while the electrolyte is one of a non-aqueous liquid electrolyte, a polymer electrolyte, and a gel-type electrolyte.
Both the negative and the positive electrodes of the present invention secondary lithium-ion rechargeable cell are less than about 0.15 mm in total thickness. This includes the thickness of the current collector and the electrode active material on the opposed current collector sides. Electrodes of a reduced thickness provide the cell with high electrode surface area relative to its volume. As such, the present cell is capable of providing pulses in excess of 30C with minimal voltage drop. Because the cell can be recharged and has high power pulse capability, it has low capacity and, therefore, minimal volume. Furthermore, self-discharge of the present invention cell is relatively low in an absolute capacity sense, which is desirable when recharging the cell from a high energy density internal power supply.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.